Interconnection of control units, sensors, and actuators, using a communication system that has a communication link, in particular a bus, and corresponding communication modules has drastically increased in the past few years in construction of modern motor vehicles, in mechanical engineering, in the machine tool industry in particular, as well as in automation. Synergetic effects as a result of the distribution of functions to a plurality of stations, in particular control units; may thus be achieved. This is known as a distributed system. Such distributed systems or networks thus have stations and the bus system or a plurality of bus systems connecting these stations. Communication between different stations increasingly takes place via such a communication system or bus system via which the data to be transferred is transmitted in messages. This communication traffic on the bus system, access and receiving mechanisms, as well as error processing are regulated via an appropriate protocol.
CAN (controller area network) has been established, for example, as a protocol in the automotive industry. This is an event-controlled protocol, i.e., protocol activities such as transmission of a message are initiated by events originating outside the communication system. Unambiguous access to the communication system, i.e., bus system, is accomplished via priority-based bit arbitration. This assumes that the data to be transmitted, and thus each message, is assigned a priority. The CAN protocol is very flexible; further stations and messages may be added without any problem as long as free priorities (message identifiers) are still available. The collection of all messages and their respective priorities to be transmitted in the network and their transmitting and receiving stations, i.e., the respective communication modules, are saved in a list, known as the communication matrix.
An alternative approach to event-controlled, spontaneous communication is the purely time-controlled approach. All communication activities on the bus are strictly periodical. Protocol activities such as the transmission of a message are triggered by the progression of a time which is valid for the entire bus system. Access to this medium is based on the assignment of time segments in which a transmitter has exclusive transmission rights. The order of messages must usually be determined before start-up. This means that a schedule is prepared which satisfies the requirements of the messages with respect to repetition rate, redundancy, deadlines, etc. This is referred to as a bus schedule. TTP/C is such a bus system, for example.
The approach of the time-controlled CAN, known as TTCAN (time-triggered controller area network) combines the advantages of both above-mentioned bus types. It meets the above-outlined requirements according to time-controlled communication, as well as the flexibility requirements to some degree. The TTCAN meets these requirements by setting up the communication round in exclusive time windows for periodic messages of certain communication stations and in arbitrating time windows for spontaneous messages of a plurality of stations. TTCAN is essentially based on time-controlled, periodic communication which is timed by a station or communication module that defines the utilization time, known as the time master, with the aid of a time reference message.
Another option for combining different types of transmission is provided by the Flex-Ray protocol, which describes a fast, deterministic, and error-tolerant bus system, in particular for use in a motor vehicle. This protocol works according to the time-division multiple-access (TDMA) method, in which the stations or the messages to be transmitted are assigned fixed time slots in which they have exclusive access to the communication link, i.e., the bus. The time slots are repeated in a pre-established cycle, so that the time at which a message is transmitted over the bus may be accurately predicted, and bus access takes place deterministically. In order to utilize the bandwidth for message transmission on the bus system, the cycle is subdivided into a static and a dynamic part. The fixed time slots are located in the static part at the beginning of a bus cycle. In the dynamic part, the time slots are dynamically defined. Exclusive bus access is enabled therein only for a short time period, referred to as minislots.
As shown, there is a plurality of different transmission technologies and thus types of bus systems. It is often required that a plurality of bus systems of the same type or different types be connected to one another. A bus interface unit, known as a gateway, is used for this purpose. A gateway is therefore an interface between different buses, which may be of the same type or different types, the gateway conveying messages from one bus to one or more other buses. Known gateways have a plurality of independent communication modules, messages being exchanged via the processor interface (CPU interface) of the particular station or the corresponding interface module of the particular communication module. This data exchange places a heavy load on the CPU interface in addition to that of the messages to be transmitted to the station itself, which, together with the resulting transmission structure, results in a relatively low data transmission rate. Furthermore, there are integrated communication controllers or communication modules which share a common message buffer, known as a message memory, and thus compensate for the structural disadvantages. However, such integrated communication modules are therefore very inflexible with respect to the data transmission and are in particular designed for a defined number of bus connections.
It is apparent that the related art has not succeeded in delivering optimum results in all respects.